The working principle of the lithium-ion battery!
Lithium-ion batteries belong to the group of batteries that generate electrical energy by converting chemical energy via redox reactions on the active materials, i.e. the negative (anode) and a positive electrode (cathode), in one or more electrically connected electrochemical cells. Lithium-ion batteries can be further divided into primary (non-rechargeable) and secondary (rechargeable) batteries, depending on whether or not they are rechargeable by applying an electric current.
In conventional lithium-ion batteries, Li+-ions are shuttled between the positive electrode (usually a layered transition metal oxide material) and a graphite-based negative electrode according to the “rocking chair” principle (cf. video).
The term discharge is used for the process in which the battery supplies electrical energy to an external load. The electrolyte in this system contains additional Li+-ions to ensure rapid transport of the ionic charge within the cell.
Besides ion conduction, the electrolyte fulfills other important purposes:
Support of the formation of effective interphases (e.g., solid electrolyte interphase, SEI or cathode electrolyte interphase, CEI) which:
- enable the battery to function
- are well Li+-ion conducting (rate!)
- are protective against further electrolyte decomposition
Contribute to cell safety – being inert to other materials, such as:
- Current collectors
- Conductive additives, Binders
- Cell casing
Step 1 - Initial state (state of charge (SOC) 0%)
When discharged, the Li+-ions are in the positive electrode material. Thus, the positive electrode is the source of the Li+-ions necessary for the conversion of electrical energy into chemical energy. To allow Li+-ions to migrate from the positive electrode to the negative electrode, the electrolyte is also enriched with Li+-ions.
Step 2 - Formation of SEI and CEI
In the very beginning of the first charging process, electrons migrate from the positive electrode material (oxidation) via an external conductor into the negative electrode material (reduction). To ensure charge neutrality, Li+-ions de-intercalate from the positive electrode material into the electrolyte and migrate through the electrolyte to the negative electrode material for subsequent storage. As a result of these reactions, boundary phases, the so-called SEI and CEI, are formed at the interfaces between electrolyte / negative electrode surface and electrolyte / positive electrode surface, respectively. These interphases are built up from insoluble electrochemically induced decomposition products of electrolyte components and Li+-ions originating from the positive electrode and enable a reversible cycling of the battery. After the formation of the SEI and CEI, further Li+-ions de-intercalate from the positive electrode material into the electrolyte and migrate through it to the negative electrode material to be subsequently incorporated into the latter.
Step 3 - Electrode reactions
After the formation of the SEI and CEI, further Li+ ions de-intercalate from the positive electrode material into the electrolyte and migrate through it to the negative electrode material to be subsequently incorporated into the latter.
LiMO2 → Li(1-x)MO2 + x·e– + x·Li+
C6 + x·e– + x·Li+ → LixC6
Overall cell reaction:
C6 + LiMO2 → LixC6 + Li(1-x) MO2
Step 4 - Colour change during intercalation / de-intercalation into graphite
Depending on the number of Li+-ions embedded in the negative electrode (depending on the state of charge, SOC), it changes colour from black over red (early SOC) to gold (100% SOC).
Step 5 - Discharge
During discharge, the reverse reactions are taking place. The electrode reactions are:
Positive electrode = “cathode” (reduction)
Li(1-x)MO2 + x·e– + x·Li+ → LiMO2
Negative electrode = “anode” (oxidation)
LixC6 → C6 + x·e– + x·Li+
Step 6 - Rocking chair principle
After discharge (SOC 0%), the Li+-ions are re-stored in the positive electrode material from which they originally came. The back and forth movement of Li+-ions reminds of the movement of a rocking chair, which is why this principle was called the “rocking chair principle”.
Especially the first cycle (charge and discharge) is associated with an irreversible loss of Li+-ions in the SEI and CEI but also in the negative electrode material. As a result, fewer Li+-ions are now able to be stored in the negative electrode in the following charge cycle, which leads to a reduced capacity of the battery.
Different aging processes take place in a lithium-ion battery, which reduces the performance of the battery over the period of use and depends strongly on the cell chemistry and the intended use of the battery. Especially the right choice of the electrolyte has an enormous influence on these aging mechanisms and underlines once more the importance of tailor-made electrolytes.
In order to optimize lithium-ion batteries with respect to the specific energy and energy density, lifetime and safety, many efforts have been made to further expand the application possibilities of LIBs. Especially the increasing demands for both high specific energy and energy density lithium-ion batteries particularly for automotive applications, raise the research efforts all over the world. The energy density and specific energy of batteries by definition is the amount of energy stored in a given system per unit volume and per unit mass, respectively. The product of the specific capacity and the mean discharge voltage gives the specific energy and this relation finds expression in equation 1:
E = C · U (1)
According to equation 1, it appears reasonable that most of the current research focuses on new positive electrode materials with higher operation voltages (high-voltage approach) and/or increased specific capacity (high-capacity approach). The high-voltage cathode materials are highly restricted by the narrow electrochemical stability window state-of-the-art carbonate-based electrolytes (≈1.0 – 4.4 V vs. Li/Li+) and reinforce the design of intrinsically stable electrolytes or suitable electrolyte additives to enable high-voltage lithium-ion batteries.